Editing Countercurrent exchange (section)

=== Countercurrent exchange in sea and desert birds to conserve water ===
Sea and desert birds have been found to have a [[salt gland]] near the nostrils which concentrates brine, later to be "sneezed" out to the sea, in effect allowing these birds to drink seawater without the need to find freshwater resources. It also enables the seabirds to remove the excess salt entering the body when eating, swimming or diving in the sea for food. The kidney cannot remove these quantities and concentrations of salt.<ref>{{Cite journal|last1=Schmidt-Nielsen|first1=Knut|last2=Fange|first2=Ragnar|date=July 1958|title=The Function of the Salt Gland in the Brown Pelican|journal=The Auk|volume=75|issue=3|pages=282–289|doi=10.2307/4081974|jstor=4081974 |issn=0004-8038|doi-access=free}}</ref><ref>{{Cite journal|last=Schmidt-Nielsen|first=Knut|date=1959|title=SALT GLANDS|url=https://www.jstor.org/stable/24944892|journal=Scientific American|volume=200|issue=1|pages=109–119|doi=10.1038/scientificamerican0159-109 |jstor=24944892 |pmid=13624738 |bibcode=1959SciAm.200a.109S |issn=0036-8733|url-access=subscription}}</ref>

The salt secreting gland has been found in seabirds like [[pelican]]s, [[petrel]]s, [[albatross]]es, [[gull]]s, and [[tern]]s. It has also been found in Namibian ostriches and other desert birds, where a buildup of salt concentration is due to dehydration and scarcity of drinking water.

In seabirds the salt gland is above the beak, leading to a main canal above the beak, and water is blown from two small nostrils on the beak, to empty it. The salt gland has two countercurrent mechanisms working in it:

a. A salt extraction system with a countercurrent multiplication mechanism, where salt is actively pumped from the blood 'venules' (small veins) into the gland tubules. Although the fluid in the tubules is with a higher concentration of salt than the blood, the flow is arranged in a countercurrent exchange, so that the blood with a high concentration of salt enters the system close to where the gland tubules exit and connect to the main canal. Thus, all along the gland, there is only a small gradient to climb, in order to push the salt from the blood to the salty fluid with [[active transport]] powered by [[Adenosine triphosphate|ATP]].

b. The blood supply system to the gland is set in countercurrent exchange loop mechanism for keeping the high concentration of salt in the gland's blood, so that it does not leave back to the blood system.

The glands remove the salt efficiently and thus allow the birds to drink the salty water from their environment while they are hundreds of miles away from land.<ref>{{cite book|last1=Proctor|first1=Noble S.|last2=Lynch|first2=Patrick J.|title=Manual of Ornithology|year=1993|publisher=Yale University Press}}</ref><ref>{{cite web|last=Ritchison|first=Gary|title=Avian osmoregulation|url=http://people.eku.edu/ritchisong/bird_excretion.htm|access-date=16 April 2011|archive-date=19 December 2019|archive-url=https://web.archive.org/web/20191219202005/http://people.eku.edu/ritchisong/bird_excretion.htm|url-status=dead}}</ref>